In accordance with the present invention a compact high fidelity multi driver point-source sound reproduction system is provided which is characterized by high efficiency, low distortion, wide bandwidth, and controlled directivity. Nearly all multi-driver loudspeakers work on the principle of acoustic summation to a point source representation at the listener's position. This can be seen in a number of common speaker configurations including home theater center channel speakers [Woofer-Tweeter-Woofer], in large format mastering speakers [Woofer-Mid-driver-Tweeter-Mid-driver-Woofer] in PA system line arrays which employ vertically stacked elements of [Woofer-High-Frequency-Source-Woofer] and in PA sound reinforcement systems where individual horns are aligned on one axis or co-axially nested, or more recently, horns that utilize multiple drivers integrated within a single horn to accomplish improved wavefront coherence and pattern control.
Most prior art related to the present invention is horn loudspeakers combining multiple drivers into a single source such as Mark Engebretson's Radiation Boundary Integrator (U.S. Pat. No. 7,134,523 B2), Thomas Danley and Bradford Skuran's Unity Summation Aperture (U.S. Pat. No. 6,411,718 B1), Richard Vandersteen's Coincident Source topology (US 20030053644 A1), Ralph Heinz's Multiple-Driver single horn loud speaker (U.S. Pat. No. 5,526,456 A) and Lee De Forest's invention titled “Improvements in or connected with sound reproducing devices” (GB 303,837 A) of 1930 which combines a high frequency reproducer and a low frequency reproducer through discrete throats which then merge into a common horn mouth.
Some of the above cited speaker systems are optimized for high sound pressure levels (horns), while others are optimized for wide dispersion (line arrays), while others are optimized for high-fidelity (mastering speakers); yet each of these loudspeaker topologies operates by summation of frequencies produced by multiple, bandwidth-specific (woofer, mid-range, tweeter, etc.), drivers used to accurately reproduce music at sound pressure levels sufficiently loud for listeners positioned at mid- or far-field and to satisfy audiences taking part in socially sanctioned rituals of induced hearing loss. In the case of multi-driver horn loud speakers, it has been possible to improve wavefront coherence beyond that of traditional discrete horn speakers, yet there remain a number of problems which limit their usefulness in professional applications.
The primary problem of multi-driver horn topologies is a “power response disparity” between the mid-range drivers and the high frequency (“HF”) source, where the HF driver often times utilized is the so-called compression driver that is located at the horn vertex. This disparity is caused, in part, by the industry standard method of injecting the mid-range frequencies through the outer horn wall with the mid-range's upper bandwidth primarily defined by the acoustic distance between the injection point and the horn vertex. Because frequencies near ½ wavelength this distance suffer self-cancellation due to reflected sound waves, (from the mid-range driver to the compression driver's diaphragm and back to the mid-range driver's position), the midrange driver exhibits significant cancellation nulls at the fundamental ½-wave thus limiting the upper bandwidth available from the midrange driver element and forcing the high frequency driver to play to lower frequencies than are mechanically or acoustically optimal.
Furthermore, the usual practice of injecting the mid-range acoustic power into the horn is accomplished using an aperture or port located on the exterior wall of the horn. The mid-range drivers are coupled into the horn wall through a so-called band-pass chamber formed by the driver's diaphragm on one side and the horn wall and aperture on the other side. This band-pass assembly then is used to “inject” acoustic power into various locations of the horn's axial expansion by variation of the band-pass injection ports. The largest problem with this band-pass injection method is that the acoustic energy entering the horn's air mass goes through the sidewall port and into a sudden 2-π steradian expansion. This sudden expansion has a large acoustic impedance discontinuity as the sound waves travel from mid-driver diaphragm, through a constricted band pass aperture and into the horn resulting in a mid-range driver which cannot efficiently couple its acoustic power into the horn's air mass (also known as “horn-loading”) at higher frequencies, which is only possible if there is a coherent pressure expansion along the horn's axis. Since both these problems affect the mid-range drivers high frequency bandwidth (and quality thereof) it has become accepted that band pass aperture-loaded mid-range drivers are not used above 1500 Hz, but more often not used above 1200 Hz-1400 Hz, thus forcing the high frequency compression driver, (located at the horn vertex), to operate into significantly lower bandwidth than would normally be advisable for high power sound reinforcement applications. Such a multi-driver horn loudspeaker relies heavily on its HF compression driver which is then forced to operate at great sound pressure level (“SPL”) which increases air turbulence and non-linear heating of the air medium within the compression driver's passages, thereby producing increasing levels of acoustic distortion, often characterized by the level of harmonic distortion. This high power requirement for the compression driver also results in increased mechanical fatigue and statistically increased failure rates as compared to compression drivers tasked with operating across less bandwidth starting at higher frequencies.
Another problem with current multi-driver horn topologies is that of wavefront interference and diffraction in arrayed (sectoral, cellular, cluster, line, etc.) speaker systems. Because of the difference between the horn's angle of acoustic expansion and the angle of its outer walls, a compromise must be made between increased mouth edge diffraction and spacing the array to form a coherent point source. This is because multi-driver horn topologies place the mid-range and bass speakers on the outside of the horn wall which makes the external enclosure geometry significantly larger than the embedded horn's acoustic geometry. For the case where forming a larger point source speaker system from a plurality of horns whose acoustic expansions are not the same angle as their external form factor, designers must separate the horns by 20 to 30 cm to make room for the bulky externally mounted drivers, and this extra separation between adjacent speakers incurs a reduced acoustic coupling between the horn mouths, and increased distortion via diffraction at the horn mouth(s). In the case where acoustic system designers couple these horn mouths together directly, the angle of co-incidence between the arrayed horns causes overlapping coverage which produces comb filtering, (overlapping bands of constructive and destructive interference which reduce signal fidelity). This is typically accounted for by adding extensions or round-overs to each multi-driver horn mouth which increases the physical dimensions of the horn or array of horns, thereby reducing their fit for use by sound-for-hire companies that transport speakers to locations and by venues where space is limited.
Yet another problem is the loss of horn directivity and loading/sensitivity at frequencies below the horn's rated low corner. In multi-driver horns, the mid-bass (150 Hz-300 Hz) and bass (10 Hz-150 Hz), bandwidth is compromised in usefulness because it does not exhibit pattern control like the majority of the horn speaker's bandwidth. Without pattern control across all frequencies reproduced, arrays will exhibit audible interference patterns. Further, without pattern control a positional amplitude response disparity between horn-uncoupled frequencies (bass and mid-bass) and horn loaded frequencies (mid-range and HF) will increase in severity as the listener's distance from the loudspeaker increases, making a flat frequency response loudspeaker which maintains relative spectral balance from near to far field impossible to achieve.
Lastly a problem known as “HF lobing” is caused by high frequency energy focusing along a horn's axis and producing an uneven or “lobed” spatio-frequency response. This results in a sound field where the frequency response changes as the listener comes directly on axis to the speakers' output. In sound reinforcement where arrays of horn speakers are used, the resulting uneven acoustic terrain becomes problematic when the audience is moving in the listening space or when the sound system is moving relative to the audience. Such HF lobing effects undermine the uniform pattern of directivity which is desirable in commercial or professional sound reinforcement applications where the listener or audience may not be stationary.
The present invention solves these problems by:                Eliminating the “power response disparity” between the mid-range drivers and HF compression driver through efficient wide bandwidth acoustic coupling between the mid-range driver and the main horn air mass;        Reducing the external footprint of the horn improving performance of point and line source arrays by locating the mid-range drivers internally to the exterior horn walls;        Defining the exterior of the horn cabinet the same as the interior acoustic expansion surface thereby allowing speakers to be accurately placed in an array next to each other without directivity or mouth diffraction issues incurred by exterior cabinet construction which makes precisely arrayed acoustic alignments impossible;        Allowing directivity to frequencies below the horns acoustic dimensions, whereby the pattern control applies to all frequencies is intentionally reproduced;        Diminishing the on axis HF and Mid-Frequency (“MF”) lobing, thereby solving the “HF lobing” problem;        Enabling mid-range drivers to be integrated at closer proximity to the central horn vertex where the compression driver is located, thereby achieving improved summation of all audible frequencies and more precise point source behavior.        